This invention relates to a process and apparatus for making press-ready lithographic printing plates having a durable, reinforced, cured image thereon wherein the image is formed and reinforced in two separate exposure steps, the first to form the image, and the second to cure or cross link the image.
Lithographic printing techniques, using, for example, anodized and silicated aluminum base plates such as described in Fromson U.S. Pat. No. 3,181,461 issued May 4, 1965, have come into widespread use in the printing industry and especially in offset printing and direct lithographic printing by newspapers using converted letterpress printing presses.
A conventional negative working lithographic printing plate of this type has a coating of a light sensitive substance that is adherent to the aluminum base sheet after exposure. If the light sensitive coating is applied to the base sheet by the manufacturer, the sheet is referred to as a "presensitized plate". If the light sensitive substance is applied to the base by the lithographer or trade plate maker, the plate is referred to as a "wipe-on-plate". Depending on the nature of the photosensitive coating employed, a coated plate may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive-acting plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative acting plate. In either case, the image area of the developed plate is oleophilic and the non-image area is hydrophilic.
In the case of a negative working plate, the surface is coated with an aqueous solution of a conventional diazo resin. The plate is dried and exposed through a negative. The exposed image areas become water insoluble and the unexposed non-image areas remain water soluble. The plate is conventionally developed with a lithographic lacquer which consists of a two-phase system, one phase containing an oleophilic resin and the other phase a hydrophilic gum. Upon application, the oleophilic resin adheres to the exposed insoluble areas, while the hydrophilic phase dissolves away the unexposed soluble non-image areas. In this way, the image areas are made oleophilic or ink receptive and the non-image areas are made hydrophilic or ink repellent.
The use of a lithographic lacquer as described above represents one way of reinforcing an image on a lithographic printing plate. Other ways have been sought to reinforce the image to make it more durable and increase the press-life of the plate. For example, in Fromson U.S. Pat. No. 3,773,514, a tough, wear-resistant solvent insoluble protective layer is applied to the light sensitive coating before imaging and developing the plate.
Another proposal is set forth by Adams et al. in U.S. Pat. No. 3,669,664 wherein a fully exposed image on a developed lithographic plate is reinforced by treating the developed plate with a lacquer made of a dispersion in water of a solution of a photopolymerizable resin in an organic solvent. The treated image can be further strengthened by exposure to actinic light and/or heat. This approach, however, is time consuming and cumbersome because it starts with a conventionally exposed and developed lithographic plate. This is an additional procedure which in fact lengthens the time required for making a press-ready plate. In the case of lithographic printing of newspapers where speed and efficiency is a critical factor this approach has serious drawbacks.
Another proposal is set forth by Bonham in U.S. Pat. No. 3,905,815 wherein a base sheet is provided with a coating of a diazo resin over which is provided a photopolymerizable layer. The latter is designed such that, for a given exposure, its rate of polymerization and insolubilization is at least as great as the rate of insolubilization of the diazo resin layer. This requires precise matching of the characteristics of the two layers and image formation and curing are carried out during exposure of the dual coated base sheet through a negative. This provides no advantage over a conventional diazo coated lithographic substrate from the standpoint of increasing the efficiency of the plate making operation.